The conversion of catalytically cracked heavy naphthas, which, typically, begin to boil within the gasoline boiling range, about 285.degree. F. (140.degree. C.), and finish boiling in the distillate range, e.g. 400.degree. F. (204.degree. C.), to materials which begin and end boiling within the gasoline boiling range, C.sub.5 to 330.degree. F., is important to refiners.
Aromatic heavy naphtha fractions such as fluid catalytically cracked (FCC) 300.degree.-425.degree. F. products are high in octane, but because they begin to boil above the gasoline end boiling range, and may contain a large proportion of sulfur impurities, they require further processing to become commercially valuable as gasoline. However, it is difficult to eliminate, or at least reduce, the properties which make them undesirable as gasoline, i.e. the high sulfur content and high boiling point, without compromising the high octane properties that make them desirable as gasoline.
Recently, it has been reported that lowering gasoline endpoint results in a product endpoint where, in a standard ASTM distillation, 90 volume percent of the gasoline distills below 300.degree. F. (T.sub.90) will reduce pollution. Meeting this T.sub.90 permits only 10% of the hydrocarbons in gasoline to boil above 300.degree. F. A significant boiling range conversion of heavy naphthas will be required to meet this goal.
U.S. Pat. No. 3,923,641 to Morrison discloses hydrocracking a C.sub.7 + naphtha over zeolite beta at moderate temperatures and pressures to achieve a high yield of iso-C.sub.4. However, the disclosure is silent on the yield and quality of higher hydrocarbons.
Upgrading a reformate by treatment with a crystalline aluminosilicate zeolite has been described. The described processes utilize the zeolite to selectively remove the normal paraffins and leave the aromatics and/or isoparaffins unchanged. U.S. Pat. No. 2,886,508 discloses contacting a reformate with a 5 angstrom unit aluminosilicate to selectively remove the normal paraffins by adsorption. U.S. Pat. No. 3,114,696 discloses cracking conditions to selectively crack the normal paraffins of a reformate. U.S. Pat. No. 3,395,094 discloses hydrocracking conditions to selectively crack the normal paraffins and also to preserve the aromatic components of the reformate feed. U.S. Pat. Nos. 3,767,568 and 3,729,409 describes treating a reformate feed over a zeolite, e.g., ZSM-5, in the presence of hydrogen, to improve the yield-octane number relationship of the reformate.
U.S. Pat. Nos. 3,945,913 and 4,078,990 both to Brennan et al. disclose production of benzene, toluene and xylenes (BTX) from alkyl aromatic feeds e.g. reformate fractions of at least nine carbon atoms over an acidic catalyst. A proposed catalyst is ZSM-5. U.S. Pat. No. 4,078,990 discloses that alkyl side chains of two or more carbon atoms rapidly dealkylate to near completion in the shallower portion of the catalyst bed which is first contacted by the feed. When this mixture, which is made up of methyl benzenes and alkanes of two or more carbon atoms, reaches the major portion of the bed, transalkylation and disproportionation reactions of the methyl benzene occur to equilibrium resulting in a product containing BTX. While this process takes advantage of the more easily dealkylated ethyl-branched aromatics of the reformate for purposes of producing methyl aromatics which can then undergo transalkylation and disproportionation reactions with the free alkyl groups, the process relies on a feed rich in ethyl-branched aromatics. There is no suggestion to utilize a feedstock which at the outset comprises methyl-branched aromatics which are more difficult to dealkylate than ethyl and higher alkyl-branched aromatics.